SUCTION FEEDING STRATEGIES OF TWO SPECIES OF MEDITERRANEAN SERRANIDAE (SERRANUS CABRILLA AND SERRANUS SCRIBA)

by

C. VILADIU1, P. VANDEWALLE2, J.W.M. OSSE3 and A. CASINOS1,* (1 Departament de Biologia Animal (Vertebrats), Universitat de Barcelona, Diagonal 645, 08028 Barcelona, Spain; 2 Universite de Liège, Institut de Zoologie, Laboratoire de Morphologie fonctionelle, 22 Quai Van Beneden, 4020 Liège, Belgium; 3 Department of Experimental Animal Morphology and Cell Biology, WageningenAgricultural University, Marijkeweg 40, 6709 PG Wageningen, The Netherlands)

ABSTRACT

A comparative study of the suction feeding strategies in Serranus cabrilla and Serranus scriba was carried out. Several specimens from both species were filmed at high speed. From these films, kinetics of the suction movements and dynamic parameters, such as volume increase/time, were calculated. A simulation model of the hydrodynamics of suction feeding was used to calculate velocities, accelerations and pressure waves. Simulation results are within the range of those previously found with experimental methods for other species. The results showed that feeding strategies are different. S. cabrilla is feeding as from an ambush, with a sudden big head expansion whereas S. scriba expands its head more gradually. The graudal increase could be related to a feeding mode in which an important component of the feeding flow is contributed to swimming. These differences are discussed in the context of the ecology of the species. KEY WORDS:Serranus, feeding, suction, simulation, ecology.

INTRODUCTION

In the last twenty years, an extensive literature on teleostean feeding mechanisms has been produced. Papers deal with different but strongly interrelated aspects of feeding, e.g., the sequence of muscle activity, kine- matics, bucco-pharyngeal pressures and hydrodynamics. Several com- prehensive articles reviewing the subject are available (see for instance, OSSE, 1969; LAUDER, 1983, 1985; MULLER et al., 1982; MULLER & OSSE, 1984; OSSE et al., 1985; AERTS, 1990).

* Address correspondence to Dr. A. Casinos: Tel.: (34) (3)402-14-54, Fax: (34) (3)403- 57-40. E-mail: [email protected]. 82

We can safely assume that evolutionary changes in head morphology are a reflection of modifications of the predatory feeding function in dif- ferent actinopterigian fishes (SCHAEFFER & ROSEN, 1961). Teleostean suction feeding can essentially be represented as a model of head expan- sion and flow. The details of the mechanism have been largely modified, however, by different ecological demands. Closely related species may have very similar morphologies and distributions and may have roughly similar food preferences. Nevertheless, they may have developed spe- cific adaptations with respect to different strategies of prey capture (see CASINOS, 1978;1981 for an example). The term ecomorphology has been used to indicate studies which elaborate on such situations (ALEXANDER, 1988; BAREL, 1983; BocK, 1990). Serranidae have a wide distribution in tropical and subtropical sea wa- ters, including the Mediterranean basin. In the latter, the genus Serranus is represented by four species. Two of them, the comber (Serranus cabrilla (Linne, 1758)) and the painted comber (Serranus scriba (Linn6, 1758)) are, in spite of their differences in coloration, very similar in many re- spects. They use to share the same coastal waters and it is possible to see them at the same rock-wall, although their preferential habitats are somewhat different. Serranus scriba prefers to live close to Po.sidonia and other marine fields of Phanerogams (the eel grass, Zostera sp.), while Ser- ranus cabrilla lives mainly among the cavities and spaces produced by fallen rock (field observation data), and also between Phanerogam fields. Both species can be found at similar depths (from 5 to 20 m), but Ser- ranus cabrilla also occurs in deeper waters (40 m, BRUSLÉ & BRUSLÉ, 1975). They show territorial and non gregarious behaviour, often trying to capture similar prey. The stomach contents of several specimens of Ser- ranus scriba (ROBINS & STARCK, 1961) show that this species catches mainly mobile preys. No adequate information is available specifically on S. cabrilla preferences from the point of view of prey mobility. Both species are synchronic hermaphrodites and their reproduction takes place during the same summer months (ROBINS & STARCK, 1961; BRUSLÉ & BRUS1.E, 1975). The external and internal morphology and the size of the two species are very similar (BENMOUNA etal., 1983, 1984a, 1984b). The main quantitative structural differences are found in the mobile parts of the splanchnocranium: (1) the suspensorium is longer in Serranus scriba, (2) the hyomandibula is more massive and its posterior joint on the skull is longer in Serranus scriba, (3) the processus ascendens of the premaxil- lae is shorter while the angle between the two arms is larger in Serranus cabrilla, (4) the adductores mandibulae, levator hyomandibulae, levator arcus palatini and stemohyoideus muscles are broader and longer in Ser- ranus scriba. Some other aspects of the head morphology and timing of